The present invention is generally related to controlled atmosphere incubators and, more specifically, to an improved incubator used to culture biological specimens.
Growing cell cultures in a laboratory incubator requires that the atmospheric conditions, such as temperature, humidity and gas concentrations, remain constant throughout the incubator. A common manner of humidifying the culturing environment or incubator chamber is to place a stainless steel pan of water in the bottom of the incubator. The water in the pan evaporates and, since the incubator requires a gas tight seal, the humidity level inside the incubator climbs to a level above 95% relative humidity. These high levels of humidity keep the cell cultures and their associated media from drying out during incubation. This is particularly critical when the volume of media is very small and the time required to culture the cells spans, for example, several days or more.
Although it is desirable to maintain these high levels of humidity for culturing and for fast humidity recovery after the incubator door is opened, it is not desirable to have condensate form anywhere inside the incubator. Condensate creates potential places for molds, spores and other unwanted bacteria to grow. Condensate will develop on any xe2x80x9ccold spotsxe2x80x9d when the temperature on a surface is below the dew point of the air/gas mixture inside the incubator. Generally, incubators operate at a temperature of 37xc2x0 C. and at elevated humidity conditions. The dew point, for example, at 37xc2x0 C. and 98% relative humidity is 36.6xc2x0 C. Therefore, any surface inside the incubator at a temperature below 36.6xc2x0 C. and in contact with the air/gas mixture will condense the water from the mixture in the form of small droplets. These may then develop into pools or puddles of condensate. It is desirable for this reason as well as others to maintain all interior surfaces at a constant temperature, however, there have been some practical limits that have required less than perfect conditions.
One location within the incubator where condensate tends to form is on the inner glass door to the incubator chamber. Generally, these doors are heated to prevent condensate from forming, especially after the door has been opened. For example, electric heaters are often placed in the outside, insulated door and heat generated by these heaters is conducted, convected and radiated through the air space between the outside, insulated door and the inner glass door. Because the heat must be transferred through the air gap between the two doors, heating of the inner glass door is relatively slow and inefficient. A more direct way of heating the inner glass door is disclosed in U.S. Pat. No. 4,039,775. This patent discloses silk screened conductive elements or lines on the glass such as are commonly used in automobile window defrosters. Problems with such silk screened window defrosting elements, however, include reduced visibility through the glass door. If these lines are especially close together, visibility is significantly reduced and if the lines are placed wider apart to increase visibility, sufficient heat may not be transferred to the glass door. Also, these conductive line elements tend to eliminate condensate only along the elements themselves, or if heated to the point that condensate is eliminated on the entire glass panel, then too much heat may be generated and the chamber may be overheated. Finally, these conductive lines can be damaged by abrasion and lose their conductive and heating capabilities.
Other problems associated with the inner glass door of laboratory incubators involve the gasket which seals the door to the perimeter of the chamber opening and the mountings used to connect the glass door to the incubator. Specifically, a gas tight seal is generally accomplished using a silicone xe2x80x9cfeatherxe2x80x9d gasket mounted around the opening of the chamber with the xe2x80x9cfeatherxe2x80x9d portion of the gasket providing a seal against the inner glass door in the closed position. To maintain the integrity of the seal, the conventional method of mounting the gasket to the chamber is by using a silicone adhesive/sealant. The gasket, also generally formed of silicone, is extruded in a profile that creates a groove for the adhesive. These gaskets are difficult to clean because of their relatively complex geometry. A particularly dirty gasket may be replaced in the field by peeling the gasket off the chamber, removing the excess silicone adhesive and attaching a new gasket in the same manner as the original one. This process, however, is tedious and requires significant down time. With respect to the door mountings, hinges are generally permanently mounted to the chamber by spot welds. As these hinges may not be removed, the direction that the door swings open and closed is determined by the side of the chamber having the hinges. Field reversible doors have been an even more significant problem in water jacketed incubators since these hinge mountings must generally be placed through the water jacket portion of the incubator.
An air circulation system is also a vital ingredient in creating the correct environmental conditions for the growth of cell cultures in a laboratory incubator. Air circulation is needed to maintain temperature uniformity within the chamber and also to effectively distribute and mix the various gases, such as CO2 and N2, used to control the pH and O2 levels within the chamber. The air flow keeps the lighter gases from stratifying within the chamber and aids in the control of CO2 and O2 levels by providing air flow across the gas sensors. A blower is generally used in conjunction with a high efficiency particulate air or xe2x80x9cHEPAxe2x80x9d filter for circulating the air and removing contaminants from the air. The HEPA filters must be maintained at a temperature above the dew point of the air mixture to prevent condensation from developing inside the filter. This condensation can restrict or block the flow of air through the filter. Problems which currently exist with such air circulation systems include the requirement for an additional heat source to maintain the temperature of the HEPA filter above the dew point of the air mixture. Also, HEPA filters have generally been mounted in locations requiring the removal of side panels and other hardware associated with the incubator in order to access the filter for replacement. As the researcher or operator may be exposed to high voltage components when removing these incubator panels, a qualified service technician must be used for what should otherwise be a simple filter replacement procedure.
While more complicated humidifying devices may be used to control the relative humidity within the chamber, the simplest device and most common method involves placing a pan of water at the bottom of the chamber and allowing the chamber to become saturated through evaporation of water from the pan. Unfortunately, this simple method of humidification is not easily controlled and the resulting fully saturated condition more easily leads to the development of condensation within the chamber.
With regard to temperature control within the chamber of the incubator, a variance in the line voltage applied to electric components which generate heat will vary the heat output of the particular electric component. Inconsistent heat output of such incubator components as heaters and motors makes it difficult to accurately and uniformly control the temperature of the incubator chamber.
Laboratory incubators simulating biological conditions also generally include carbon dioxide sensors to regulate the amount of CO2 within the chamber and thereby simulate a specific pH or acidity level. Two general types of CO2 sensors are sensors based on a thermal conductivity detection and sensors utilizing infrared technology. With respect to thermal conductivity CO2 sensors, while these sensors are generally less costly, they are also sensitive to humidity and oxygen levels and to temperature variations within the chamber. While certain compensation systems have been proposed, these systems have not entirely solved the problems with environmental sensitivities. Infrared sensors are much less sensitive to the above noted environmental conditions. However, calibration requires the use of a tank of gas having a specific percentage of CO2.
In view of the above noted problems and deficiencies of incubators in general, there is a need for an incubator which provides a more accurate simulated chamber condition and which is more easily operated and maintained in the field by the end user.
The present invention is directed to an incubator which, in a first aspect, includes a double door construction wherein one door comprises an outside insulated door and the second door is an inside glass door for alternatively sealing and accessing the incubator chamber. In accordance with the present invention, the glass door is directly heated by an electrically conductive, clear coating placed on at least one surface of the door. The coating also has the characteristic of causing the glass to have low emissivity.
Specifically, a dual glass pane configuration is used having two panes of glass separated by a space and with one pane being coated and electrically heated. A pair of bus bars are disposed on opposite sides of the glass door and provide for electric conduction across the coated surface. This glass door therefore provides direct and complete heating of the glass to prevent or remove condensate on the inside surface of the glass without the disadvantages associated with indirect heating methods or typical silk screened conductive lines.
The glass door is sealed by a unique gasket which may be removed for cleaning and sterilization and replaced in a simple operation. Specifically, the gasket includes a mounting portion and an outwardly extending feather portion which seals against the inside glass surface. The mounting portion simply presses onto the perimeter or edge of the chamber opening with a friction fit. The feather or sealing portion of the gasket extends outwardly in a manner and direction which prevents buckling of the feather portion when mounted around the curved corners of the door opening.
In accordance with a further aspect of the invention, the glass door is uniquely mounted onto the front water jacket portion of the incubator to allow the door to be easily reversed in the field from left to right swinging or vice versa.
In accordance with another aspect of the invention, the air flow pattern within the incubator is created by a blower assembly mounted within the incubator chamber in an easily accessible manner. Air is pulled into a blower near the top of the chamber and exhausted through duct work that runs across the top of the chamber, down a plenum located behind a shelf support panel in the chamber and across the bottom of the chamber until the air disperses and is pulled up vertically through perforated shelves located inside the chamber. In accordance with the invention, a HEPA filter is mounted directly to the blower and is located internally to the chamber. Therefore, the blower assembly does not require an additional heat source to maintain its temperature above the dew point of the air mixture within the chamber. The HEPA filter is also easily removed and replaced by a researcher or other user from within the chamber and does not require the removal of side panels or other hardware which might involve exposure to high voltage wiring and/or components.
The invention further contemplates a control volume approach to regulating the humidity level within the chamber. Specifically, relatively low humidity ambient air is drawn into the inlet of the blower through a filter and, at the same time, air exits the chamber through a filtered outlet. This air exchange prevents complete saturation of the incubator chamber and therefore assists in preventing the formation of condensation inside the incubator. HEPA filters are preferably used to filter both the incoming ambient air as well as the air exiting the chamber. This minimizes the potential for introducing contamination into the incubator and allowing escape of contamination from the incubator. As this air exchange system is formed as part of the internal air circulation system in this unique manner, no additional components or electronics are necessary for controlling the relative humidity within the chamber.
The motor for operating the blower is mounted directly above and against the top of the incubator chamber. The control of the present invention operates to maintain constant power, and therefore constant heat generation, from the motor during voltage and frequency variations. Specifically, the invention features a method and apparatus for controlling application of a line voltage to an electric motor in the chamber, to reduce variations in heat produced by the motor that might be caused by line voltage variations. In accordance with this aspect, the line voltage and/or frequency is measured and, based on performance characteristics of the motor, the line voltage is applied to the motor in a pattern which will result in a predetermined net root-mean-square voltage across the motor, and consequently a predetermined amount of heat.
In particular embodiments, the line voltage is an AC line voltage, applied to the motor through a triac. The triac is activated a variable delay time after a zero crossing of the line voltage (and automatically deactivates when the motor current reaches zero). The appropriate delay time for a given RMS line voltage is determined by applying various line voltages and frequencies, and various delay times, to determine a delay time for each line voltage and frequency which results in a predetermined RMS voltage across the motor""s terminals. This delay time is then stored in the table for later retrieval and use in controlling the motor.
In accordance with another aspect, the operation of a heater under the control of a closed-loop temperature control system is improved by reducing variation in the heater""s heat output caused by line voltage variation. The closed-loop temperature control is calibrated for operation at a predetermined line voltage, such as 90 Volts RMS, and in operation generates a heater power fraction indicating the fraction of full heater power to be applied to the chamber. The control circuit determines a ratio of the root-mean-square amplitude of the measured line voltage to (90)2. Then, a revised heater power fraction is obtained by dividing the heater power fraction demanded by the closed-loop temperature control system by the computed ratio. The heater then generates the revised heater power fraction of its maximum heat output. As a result, the closed-loop temperature control system will obtain a consistent heat output from the heater independent of variations in line voltage.
In preferred embodiments, the line voltage is an AC line voltage which is alternately applied, and not applied, to the heater, such that power is applied to the heater for a fraction of time equal to the revised heater power fraction.
In accordance with another aspect, a gas sensor such as a thermal conductivity carbon dioxide sensor is calibrated to compensate for sensitivities to oxygen and/or humidity, by measuring the temperature and humidity and/or oxygen in the vicinity of the gas sensor, and then determining a humidity and/or oxygen variation between said the current humidity/oxygen level and the humidity/oxygen level extant when the gas sensor was calibrated. Next, the incubator temperature is used to determine an amount of humidity and/or oxygen variation which would cause a one percent apparent change in gas concentration indicated by the gas sensor. Then the humidity and/or oxygen variation is divided by the amount of humidity/oxygen variation which would cause a one percent apparent change in gas concentration, and the resulting value is added to the gas concentration indicated by the gas sensor, thus compensating the gas sensor reading for variations in humidity and/or oxygen.
In preferred embodiments, the humidity sensor measures a relative humidity level, and this relative humidity level is converted to grains based on the measured temperature, so that the grains may be used to compute the apparent change in gas concentration caused by humidity variation.
In another aspect, the invention features a circuit for detecting whether an incubator door is open. The door has a conductive frame attached thereto which is electrically isolated from the grounded frame of the chamber. When the door is closed, the conductive door frame contacts the chamber frame and the door frame is thereby grounded. However, when the door is open, no such contact is made, so that a pull-up circuit electrically connected to the door frame pulls the door frame up to a logic xe2x80x9chighxe2x80x9d voltage (e.g., 5 volts) which can be detected by logic circuitry in the controller and used to indicate that the door is open.
In preferred embodiments, the logic circuitry includes electrostatic isolation circuitry which protects the controller from electrostatic discharge in the door conductor.
An optional infrared based carbon dioxide sensing element or detector is also contemplated by the present invention. In accordance with the invention, a unique calibration method eliminates the need for a separate supply of calibrating gas. This calibration procedure takes advantage of uniform ambient air conditions wherein CO2 represents 0.033% of the volume of ambient air. Specifically, a predetermined concentration of gas is applied to the sensor, and then the sensor output is measured when the normal power is applied to the light source, and when a reduced power level is applied to the light source. These two measurement points are then curve-matched to a known, predetermined curve of source power vs. sensor output for the sensor, to produce an offset and gain factor to be applied to further sensor readings.
Further objects and advantages of the present invention will become more readily apparent to those of ordinary skill upon review of the following detailed description taken in conjunction with the accompanying drawings.